OK, so a friend of mine had to abandon the flat she'd used for years without any issues because of another flat in the same building being indiscreet. My friend had a 2 bed and she'd often rent the other bedroom. Never had a problem.
But one day she realised that there was another working flat on the same floor, right next to the lift. When she was coming from or waiting from the lift, she'd notice men standing in front of the flat and knocking. And waiting and knocking again. Then after a few minutes there was the tap-tap of heels and the door would open and a woman's voice saying hello to let him in.
I saw this, myself, on a few occasions when visiting my friend. Different men of all ages, classes and races, waiting at the same door.
1. Who wears high heels indoors and not to go out?
2. Who has all male visitors at all hours?
3. Who has visitors to their homes of all different races, ages and social classes? Most people tend to socialise with people like themselves.
Because they were right next to the lift, quite a lot of people saw this. Because they made the clients stand out there knocking, everyone got a good look.
So the police got involved and CCTV was studied. And that's when they noticed the second stream of men going on down to my friend's flat. Hers was around a corner and much less viewable, but if you watch the CCTV you'll see a parade of men coming and going. Watch the CCTV for that corridor and you've identified the flat.
So even though my friend was always discreet and went undetected for years, once someone had a look, it was spotted. And they started looking because the other flat might as well have had a red light over the door.